You would still need to pay the bank the full 700k even if it's only worth 300k now. This might mean that you still owe 400k on a 300k asset. In this way you can be underwater while still being a 30% owner.
But that option is obviously worse than declaring bankruptcy, which you can do. You can't be forced into remaining with your underwater house.
But that option is obviously worse than declaring bankruptcy, which you can do. You can't be forced into remaining with your underwater house.